1. Field of the Invention
The present invention relates to a three-phase brushless motor, and more particularly to a structure of stator teeth relative to permanent magnet rotor poles.
2. Description of Related Art
An example of a conventional brushless motor 1 is schematically shown in FIG. 2. The brushless motor 1 is composed of a stator 2 and a rotor 6 rotatably supported inside the stator 2. The stator 2 includes an outer core ring 3, an inner core ring 4 having nine teeth 4a-4i and connected to the outer core ring 3, and three-phase coils wound around respective teeth. The three-phase coils consist of U-phase coils 5U (5U1, 5U2 and 5U3), V-phase coils 5V (5V1, 5V2 and 5V3) and W-phase coils 5W (5W1, 5W2 and 5W3). The nine teeth 4a-4i extend in the radial direction from the inner core ring 4 with an equal central angle of 40.degree. between two neighboring teeth. The U-phase coils 5U1, 5U2 and 5U3 are respectively wound around the first tooth 4a, the second tooth 4b and the third tooth 4c. Similarly V-phase coils 5V1-5V3 are respectively wound around the fourth to sixth teeth 4d-4f, and W-phase coils 5W1-5W3 are respectively wound around the seventh to ninth teeth 4g-4i. The coils around the first, third, fourth, sixth, seventh and ninth teeth are wound in a first direction, and other coils around the second, fifth and eighth teeth are wound in the second direction which is opposite to the first direction. After all the coils are wound on the respective teeth of the inner core ring 4, the inner core rings 4 is connected to the outer core ring 3 as shown in FIG. 2. The stator 2 thus assembled is housed in a housing (not shown).
The rotor 6 composed of a shaft 7 and a permanent magnet 8 is rotatably housed in the housing and held inside the stator 2 with a certain air gap therebetween. The permanent magnet 8 is magnetized into eight poles, an N pole and an S pole being alternately formed with an equal central angle of 45.degree. between two neighboring poles.
Three-phase alternating excitation current consisting of U, V, and W phases having 120.degree. phase difference from each other is supplied to the respective coils, i.e., the U-phase coils 5U, the V-phase coils 5V and the W-phase coils 5W. A rotating magnetic field is generated in the stator 2 by the three-phase alternating excitation current, and thereby the rotor 6 rotates relative to the stator 2. As disclosed in JP-B2-8-8764, if the number of magnet poles P is set to 8n (n is an integer equal to or larger than 1), and the number of teeth T is set to 9n, a ripple frequency of a cogging torque becomes relatively high. Since the amount of the cogging torque is inversely proportional to the ripple frequency, it is possible to reduce the cogging torque and thereby to realize a low vibration motor by thus selecting the number of magnet poles P and the number of teeth T. In the example shown in FIG. 2, n is one, and therefore P=8 and T=9.
In the stator structure shown in FIG. 2, the center of a tooth does not always coincide with the center of a magnet pole. For example, when the center of the second tooth 4b coincides with the center of an N-pole as shown in FIG. 2, the centers of the first tooth 4a and the third tooth 4c shift from the centers of respective S-poles. This results in phase shifting among voltages V1, V2 and V3 induced in respective coils 5U1, 5U2 and 5U3 of the U-phase, as shown in FIG. 3. That is, the phase of V1 is advanced from the phase of V2 by 20.degree. in electrical angle, and the phase of V3 is delayed from the phase of V2 by 20.degree. in electrical angle. Therefore, though a peak voltage of each voltage V1, V2 and V3 is 1.0, a composite voltage V0 does not reach 3.0, but it only reaches 2.879. As shown in FIG. 4, the level of the composite voltage V0 becomes lower as the phase shift among V1, V2 and V3 becomes larger. The same is similarly applied to other phases, the V-phase and the W-phase. Accordingly, the excitation current supplied to the stator coils is not effectively utilized for generating the rotating field in the conventional motor. This has been a problem in realizing a high efficiency motor in a compact size.